Acid catalysts are utilized in the chemical and biochemical industries to conduct a wide range of chemical transformations. A range of homogenous and heterogeneous catalysts are used, some of which require high temperatures to be effective and produce considerable amounts of bi-products and waste. These unwanted products and waste have to be treated and destroyed. The drive for more environmentally friendly processes—“Green Chemistry”—highlights the need for reusable, more effective and selective catalysts. This need has led to investigations into the design of new materials which can catalyze a variety of chemical transformations. Key requirements for such new catalysts are very good thermal stability, high insensitivity to chemical attack, high functional group loading, fixed and rigid structures, optimum functional groups so as to avoid rearrangements and side products, limited swelling capability, insolubility in organic solvents, ease of purification and high reusability, high ageing resistance and ease of access to the functional group which conducts the chemical transformation. In addition, the processes to make such catalyst systems have to be flexible so as to enable the production of optimum structures and shapes for specific reactions. This could include tailoring the porosity from anywhere between macroporous to microporous structures, variable loading of the functional group, ease of making different metal derivatives and selective pH ranges.
A range of metals and catalysts have been embedded within or adsorbed onto the surface of silica, and other materials. One of the problems encountered with these systems is the loss of the active functional groups due to their often weak attachment to the silica. New organo-silica materials are needed which while possessing the properties described above, have functional groups which are strongly attached and which bind strongly to a range of metals and catalysts. As a consequence of stricter environmental regulations, there is a growing requirement for more effective systems for the removal and recovery of metals from a wide spectrum of metal contaminated solvents and aqueous-based wastes, and from contaminated waters. For example, industries such as the nuclear industry and the electroplating industry generate substantial quantities of water-based effluent, which are heavily contaminated with undesirable metal ions. Cation exchangers have been used to remove metal ions from solution. The type of cation exchangers which are employed are primarily of an organic, partly cross-linked polystyrene backbone with sulfonate groups attached to some of the phenyl rings. The physical and chemical properties of these polystyrene sulfonic cation exchangers are strongly affected by the organic nature of the polymeric backbone, so that a number of disadvantages affect their technical field of application. These limitations include relatively low temperature resistance (100-130° C.), sensitivity to chemical attack which can result in complete breakdown of the polymer matrix, strong swelling capacity, non-usability in certain organic solvents and the need for swelling to make the functional groups accessible. Organophosphonic acid cation exchangers have also been reported in e.g., U.S. Pat. No. 5,281,631. These systems are based on the products from the copolymerization of vinylidene disphosphonic acid with styrene, acrylonitrile and divinylbenzene. However, the physical and chemical properties of these organophosphonic acid resins are very similar to the polystyrene sulfonic acid based systems, and thus their field of application is limited.
Inorganic polymer systems such as silica, aluminum oxide and titanium oxide, which do not suffer some of these drawbacks, have been investigated as ion exchangers. Active functional groups or metals are attached by a variety of means to these systems. However, these systems suffer from the fact that only a low level of functional groups can be bound onto these surfaces. One of the additional problems encountered with these systems is that the functional groups can be removed on use or on standing. This is due to the rather weak attachment between the functional group and the surface atoms on the support. Strong acidic cation exchangers based on sulfonic acid groups attached to an organopolysiloxane backbone have been described in U.S. Pat. Nos. 4,552,700 and 5,354,831. The materials reported have a general formula of (O3/2Si—R1—SO3)xMx, where R1 is an alkyl or cycloalkyl fragment, M is hydrogen or a mono to tetravalent metal ion, and where the free valences of the oxygen atoms are saturated by silicon atoms of other groups of this formula and/or by cross-linking bridge members such as SiO4/2, R1SiO3/2, TiO4/2, AlO3/2, etc. While these materials can act as cation exchangers, it is generally recognized that sulfonic acid groups are limited in their effectiveness to complex with a range of metals and in comparison to other functional groups. In addition, the sulfonate group is also limited by the fact that it is a mono anion, and thus more of these functional groups are needed to bind to metals compared to other functional groups.